Today, in industry there are many engineering plastics with a broad range of properties. For some applications, the engineer may want a combination of properties inherent in several different types of plastics. One way to obtain these different properties is to develop techniques that will combine these materials while maintaining their desirable qualities. Coextrusion of multilayered sheet and film is such a technique. Some of the more desirable material properties are good corrosion, light, or temperature resistance, gas/moisture permeability with organoleptics (aroma/taste barrier properties), high tensile strength, high elongation and desirable electrical properties. It is also a necessary requirement that, after forming, multilayer polymer sheets should not delaminate in use. Good bonding between the two polymers is often achieved by holding them together under pressure in the molten state. This can be accomplished by coextruding the polymer sheet from a single die orifice.
The ultimate theoretical strength properties of polymeric materials are generally calculated based on a state in which the molecules are fully extended and are perfectly aligned and packed to maximize the forces of intermolecular attraction. This ideal condition is never achieved in practice and the properties expressed by a particular fabricated form of a polymeric material are some (usually small) fraction of the theoretical as determined by the extent to which the fully extended, perfectly aligned state is approached under the conditions imposed by the particular method of fabrication. In the case of polyolefins, and specifically for polypropylene, the theoretical modulus is calculated to be about 7 million psi, representing a theoretically achievable draw ratio of around 100. For existing commercial processes, such as that for producing monoaxially drawn strapping tape, ribbon, or fibrillated fiber, operation at economically high rates (&gt;30 meters/min) with practical feedstocks of conventional types of polypropylene gives rise to draw ratios of 6 to 8 and modulii of 400-500 thousand psi. Attempts to develop higher draw ratios and properties lead to tape breakage and/or fibrillation.
The production of high modulus films and fibers by various methods of deformation of thermoplastics has become commonplace in industry. There are many methods which are currently used for producing high modulus and high strength films and fibers in industry. Deformation processes such as roller-drawing, hydrostatic extrusion, solid state extrusion, gel spinning/hot drawing, superdrawing and zone-annealing have been employed to try and achieve the highest possible strength and modulus. The gel spinning/hot drawing technique currently produces the highest strength and modulus fibers of polyethylene. The modulii of these fibers have been reported to be as high as 120 CPa (17.4 MM psi) with a breaking tensile strength just over 4.0 GPa (580M psi). A similar technique, drawing dry gels crystallized from dilute solution, currently produces the highest strength and modulus polypropylene films with a Young's modulus of 36 GPa (5.2 MM psi) and a tensile strength of 1.08 GPa (146M psi).
Gel spinning and drawing dry gels are excellent methods for producing high strength fibers and films but they have a number of inherent problems. Generally speaking, the techniques are a high technology method requiring specific polymer molecular weights and distributions. In addition, the gel spinning technique requires extensive solvent removal schemes such that the costs for these polyolefin fibers are comparable to acid spun high temperature aromatic polyamides. Methods which avoid solvents typically produce materials with significantly lower mechanical properties and/or the rate of the production does not offer much hope of being commercial in the forseeable future.
On the other hand, recent patent and open literature publications describe other ways to approach much closer to the theoretical limits. Those which develop the highest strength in polyolefins are based on complex and elaborate processes and require non-conventional, expensive, ultra-high molecular weight forms of the polyolefin feedstock. The processes in this category are:
(1) Process for drawing mats of solution-grown crystals of ultra-high molecular weight polyolefins, reported by R. S. Porter, et al, Polym. J., 15, 327 (1983)., ibid., 16, 75 (1984); and by Furuhata, et al, J. Polym. Sci.: Polym. Phys. Ed., Vol. 22, 133 (1984). PA1 (2) Process for high-pressure solid-state extrusion of specially formed billets of ultra-high molecular weight polyolefins, reported by Porter, et al, Polym. Eng. Sci., 18, 861 (1978). PA1 (3) Complex, multi-stage processes (discussed above) for the preparation and drawing of gel forms of ultra-high molecular weight polyolefins as claimed by Allied Corporation, U.S. Pat. No. 4,413,110, Nov. 1, 1983; by DSM, C and EN, Feb. 6, 1984, p. 17; and by General Technologies Applications, Advanced Materials, Vol. 5, No. 19, Nov. 14, 1983. PA1 (1) Processes for solid-state ultra-drawing polyethylene and polypropylene hydrostatically under high pressure (500-1000 bar) claimed by Meier and Jarecki, U.S. Pat. No. 4,348,350, Sept. 7, 1982. PA1 (2) Roller-drawing processes described by Kaito, et al, J. Appl. Polym. Sci., Vol. 30, 1241 (1985); and by Woodhams et al, paper presented at the Chemical Institute of Canada Conference, Kingston, Ontario, June 3-5, 1985. PA1 (1) A recently introduced process of this kind, called BeXor, claims a particularly good balance of strength and toughness in thick-wall cylinder or sheet forms imparted by a method of solid-state hydrostatic extrusion; Kusan, Inc., U.S. Pat. No. 4,282,277. PA1 (1) Method for manufacturing polypropylene strap in a manner forming a gradation of cross sectional draw ratios from low at the surface to high at the center., Tsukasa Kasei Kogyo, U.S. Pat. No. 4,451,524, May 29, 1984. PA1 (2) Zone-annealing method for preparation of ultra-high strength fiber; Kunugi, New Materials and New Processes, Vol. 1, 58 (1981).
A second category of processes consists of processes that are claimed to utilize conventional commercial types of polyolefins as feedstocks (i.e., they are not dependent on expensive and generally unavailable ultra high molecular weight forms). These processes are reported to develop levels of strength and stiffness that are high, but more modest than developed by the schemes in the first category. The methods employ conditions and hardware that are complex and/or are not representative of conventional plastics processing systems in use or commercially available today nor have any of the approaches been demonstrated to achieve the highly extended state at any but very low rates of processing (although it is generally implied that high processing rate will be attainable with further development of methodology or equipment). Processes in this category are:
A third process category comprises a variety of solid phase stretch-forming and pressure-forming and forging methods. These generally produce substantially enhanced tensile strength and toughness, but give only moderate increases in stiffness. Some have been in limited commercial use for a decade or more. One example is:
A fourth category of solid-state processing is represented by two systems which adopt the approach of attempting to tailor the micro- and macro- features of the morphology of fiber or tape forms of polyolefins in manners which facilitate their drawability. They utilize conventional commercial varieties of polyolefins to produce polymer forms which exhibit substantially improved drawability in subsequent solid state stretching. The systems referred to are:
We have a method for producing films with similar properties through tensile drawing with a heating device. It is desirable that such a process be both continuous and applicable to either thin or thicker sheets. Certainly our method can be readily modified to accomplish this goal. Further, control of the draw ratio is possible by adjusting the draw velocity and/or the draw temperature.
The concept of mutual mechanical reinforcement was initially conceived in terms of a high elongation material helping a high modulus low elongation material to undergo large ductile deformation before failing. Reportedly, layers of high modulus low elongation material were sandwiched between strongly adhering layers of high elongation material. The high elongation layers operate to prevent the propagation of transverse cracks across the high modulus layers. With crack propagation blocked, the high modulus low elongation layers become more ductile and all layers can deform simultaneously to large elongations. This allows the composite film to have both high modulus and high elongation.
Unfortunately, not all combinations of high elongation polymers and brittle polymers exhibit this behavior. Many times, a situation of "mutual interlayer destruction" is observed, i.e., the brittle polymer fails at its characteristic low elongation and causes a simultaneous, premature failure of the high elongation layers. See "Mulilayer plastic films" by W. J. Schrenk, Applied Polymer Symposium No. 24, pp. 9-12 (1974). Other related articles published by Dow include "Some Physical Properties of Multilayered Films", by W. J. Schrenk and T. Alfrey, Jr., Polymer Engineering and Science, Vol. 9 No. 6, pp. 393-399 (November 1969) and "Mechanical Interactions in Laminated Sheets. II", by Sudershan, K. Bhatega and Turner Alfrey, Jr., J. Macromol. Sci.-Phys. B 19(4), pp. 743-771 (1981).
The Dow publications describe the phenomenon of "mutual mechanical reinforcement in multilayer films" for systems which comprise thin layers of high modulus, low elongation material sandwiched between thin, adhering layers of highly elongatable material. When such a material is tested to failure in tension, it is observed that the high modulus material undergoes large ductile deformation before failing, much in contrast to the tensile behavior of the same material as a monolithic film. Example systems are PET/Aluminum Foil/PET, polyethylene/polystyrene/polyethylene, and polypropylene/polystyrene/polypropylene.
While the result in the Dow laminates is in part the same, i.e. enhanced extensibility of a component of a laminar composite over the extensibility of that component alone, the materials property sequence in the construction of our laminate is entirely different from the Dow systems (high elongation, low modulus material sandwiched between layers of low modulus, low elongation material, versus, high modulus, low elongation material sandwiched between layers of high elongation material) and the total synergistically-enhanced extensibility of our system is missing in the Dow composites. Accordingly, our observed results are not at all expected or predictable based on this prior art.